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Feature Design of RSVP Support for RTP Header Compression, Phase 1

Network administrators use RSVP with Voice over IP (VoIP) to provide quality of service (QoS) for voice traffic in a network. Because VoIP is a real-time application, network administrators often configure compression within the network to decrease bandwidth requirements. Typically, compression is configured on slow serial lines (Figure 1), where the savings from reduced bandwidth requirements outweigh the additional costs associated with the compression and decompression processes.

Note RTP header compression is supported by Cisco routers.

Figure 1 Configuring Compression

Originating applications know if their traffic is considered compressible, but not whether the network can actually compress the data. Additionally, compression may be enabled on some links along the call's path, but not on others. Consequently, the originating applications must advertise their traffic's uncompressed bandwidth requirements, and receiving applications must request reservation of the full amount of bandwidth. This causes routers whose RSVP implementations do not take compression into consideration to admit the same number of flows on a link running compression as on one that is not.

Predicting Compression within Admission Control

Network administrators, especially those whose networks have very low speed links, may want RSVP to use their links as fully as possible. Such links typically have minimum acceptable outgoing committed information rate (minCIR) values between 19 and 30 kbps. Without accounting for compression, RSVP can admit (at most) one G.723 voice call onto the link, despite the link's capacity for two compressed calls. Under these circumstances, network administrators may be willing to sacrifice a QoS guarantee for the last call, if the flow is less compressible than predicted, in exchange for the ability to admit it.

In order to account for compression during admission control, routers use signalled Tspec information, as well as their awareness of the compression schemes running on the flow's outbound interfaces, to make local decisions as to how much bandwidth should actually be reserved for a flow. By reserving fewer resources than signalled by the receiver, RSVP can allow links to be more fully used.

Benefits of RSVP Support for RTP Header Compression, Phase 1

Additional Calls Accommodated on the Same Link

The RSVP Support for RTP Header Compression, Phase 1 feature performs admission control based on compressed bandwidth so that additional voice calls can be accommodated on the same physical link.

Perform this task to verify that the RSVP Support for RTP Header Compression, Phase 1 feature is functioning.

SUMMARY STEPS

1. enable

2. show ip rsvp installed[detail]

3. show ip rsvp interface [interface-typeinterface-number] [detail]

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2

show ip rsvp installed [detail]

Example:

Router# show ip rsvp installed detail

Displays information about interfaces and their admitted reservations and the resources needed for a traffic control state block (TCSB) after taking compression into account.

•The optional detail keyword displays the reservation's traffic parameters, downstream hop, compression, and resources used by RSVP to ensure QoS for this reservation.

Step 3

show ip rsvp interface [interface-typeinterface-number] [detail]

Example:

Router# show ip rsvp interface detail

Displays information about interfaces on which RSVP is enabled, including the current allocation budget and maximum available bandwidth and the RSVP bandwidth limit counter, taking compression into account.

Examples

Sample Output for the show ip rsvp installed detail Command

In this example, the show ip rsvp installed detail command displays information, including the predicted compression method, its reserved context ID, and the observed bytes saved per packet average, for the admitted flowspec.

Sample Output for the show ip rsvp interface detail Command

In this example, the show ip rsvp interface detail command displays the current interfaces and their configured compression parameters.

Router# show ip rsvp interface detail

Et2/1:

Bandwidth:

Curr allocated: 0 bits/sec

Max. allowed (total): 1158K bits/sec

Max. allowed (per flow): 128K bits/sec

Max. allowed for LSP tunnels using sub-pools: 0 bits/sec

Set aside by policy (total): 0 bits/sec

Admission Control:

Header Compression methods supported:

rtp (36 bytes-saved), udp (20 bytes-saved)

Neighbors:

Using IP encap: 0. Using UDP encap: 0

Signalling:

Refresh reduction: disabled

Authentication: disabled

Se3/0:

Bandwidth:

Curr allocated: 0 bits/sec

Max. allowed (total): 1158K bits/sec

Max. allowed (per flow): 128K bits/sec

Max. allowed for LSP tunnels using sub-pools: 0 bits/sec

Set aside by policy (total): 0 bits/sec

Admission Control:

Header Compression methods supported:

rtp (36 bytes-saved), udp (20 bytes-saved)

Neighbors:

Using IP encap: 1. Using UDP encap: 0

Signalling:

Refresh reduction: disabled

Authentication: disabled

Troubleshooting Tips

The observed bytes-saved per packet value should not be less than the configured or default value. Otherwise, the flow may be experiencing degraded QoS. To avoid any QoS degradation for future flows, configure a lower bytes-saved per packet value.

Flows may achieve less compressibility than the default RSVP assumes for many reasons, including packets arriving out of order or having different differentiated services code point (DSCP) or precedence values, for example, due to policing upstream within the network.

If compression is enabled on a flow's interface, but the compression prediction was unsuccessful, the reason appears in the output instead of the reserved compression ID and the observed bytes-saved per packet.

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Glossary

admission control—The process in which a Resource Reservation Protocol (RSVP) reservation is accepted or rejected based on end-to-end available network resources.

bandwidth—The difference between the highest and lowest frequencies available for network signals. The term also is used to describe the rated throughput capacity of a given network medium or protocol.

compression—The running of a data set through an algorithm that reduces the space required to store or the bandwidth required to transmit the data set.

DSCP—differentiated services code point. The six most significant bits of the 1-byte IP type of service (ToS) field. The per-hop behavior represented by a particular DSCP value is configurable. DSCP values range between 0 and 63.

flow—A stream of data traveling between two endpoints across a network (for example, from one LAN station to another). Multiple flows can be transmitted on a single circuit.

flowspec—In IPv6, the traffic parameters of a stream of IP packets between two applications.

G.723—A compression technique that can be used for compressing speech or audio signal components at a very low bit rate as part of the H.324 family of standards. This codec has two bit rates associated with it: 5.3 and 6.3 kbps. The higher bit rate is based on ML-MLQ technology and provides a somewhat higher quality of sound. The lower bit rate is based on code excited linear prediction (CELP) compression and provides system designers with additional flexibility. Described in the ITU-T standard in its G-series recommendations.

packet—A logical grouping of information that includes a header containing control information and (usually) user data. Packets most often refer to network layer units of data.

QoS—quality of service. A measure of performance for a transmission system that reflects its transmission quality and service availability.

router—A network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.

RSVP—Resource Reservation Protocol. A protocol that supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature (bandwidth, jitter, maximum burst, and so on) of the packet streams they want to receive.

RTP—Real-Time Transport Protocol. A protocol that is designed to provide end-to-end network transport functions for applications transmitting real-time data, such as audio, video, or simulation data, over multicast or unicast network services. RTP provides such services as payload type identification, sequence numbering, timestamping, and delivery monitoring to real-time applications.

TCSB—traffic control state block. A Resource Reservatiion Protocol (RSVP) state that associates reservations with their reserved resources required for admission control.

Tspec—Traffic specification. The traffic characteristics of a data stream from a sender or receiver (included in a Path message).

UDP—User Datagram Protocol. A connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.

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Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.